Butterfly wings exhibit spatial variation in chromatin accessibility

Butterfly wings exhibit a diversity of patterns which can vary between forewings and hindwings and spatially across the same wing. Regulation of morphological variation involves changes in how genes are expressed across different spatial scales which is driven by chromatin dynamics during development. How patterns of chromatin dynamics correspond to morphological variation remains unclear. Here we compared the chromatin landscape between forewings and hindwings and also across the proximal and distal regions of the hindwings in two butterfly species, Bicyclus anynana and Danaus plexippus. We found that the chromatin profile varied significantly between the different wing regions, however, there was no clear correspondence between the chromatin profile and the wing patterns. In some cases, wing regions with different phenotypes shared the same chromatin profile whereas those with a similar phenotype had a different profile. We also found that in the forewing, open chromatin regions (OCRs) were AT rich whereas those in the hindwing were GC rich. GC content of the OCRs also varied between the proximal and hindwing regions. These differences in GC content were also reflected in the transcription factor binding motifs that were differentially enriched between the wings and wing regions. Our results suggest that distinct wing patterns may result from the interaction of pioneer factors, including Hox genes, differentially opening chromatin in different wings and wing regions and cooperating with other transcriptions factors, that show preferences for specific GC content, to function either as activator or repressors of nearby genes.